(1) Field of the Invention
The present invention relates to a process for the preparation of cellulose ester films excellent in optical characteristics and thickness uniformity. More particularly, the present invention relates to a process for preparing a film from a solution of a cellulose ester such as nitrocellulose by the rotational film-forming method. Moreover, the present invention relates to a photomask-protecting optical film obtained according to this process.
(2) Description of the Prior Art
Since a film of a cellulose ester such as nitrocellulose is used as an optical film for protecting a photomask such as LSI or a reticule from dusts, development of a process capable of forming a cellulose ester film free of color nonuniformity or streaks and having good optical characteristics is desired.
As means for preparing such a cellulose ester film, there has been adopted the so-called rotational film-forming method in which a cellulose ester is dissolved in an organic solvent, the solution is supplied to the horizontal surface of a rotary member and the solution is expanded and formed into a film by a centrifugal force generated by rotation of the rotary member. As typical known instances of this method, there can be mentioned (1) a process in which nitrocellulose is dissolved in an alcohol, a ketone or a polyglycol such as 1,2-dimethoxyethane and a film is prepared from the solution by the rotational film-forming method (see Japanese Patent Application Laid-Open Specification No. 196501/83), (2) a process in which a cellulose ester, specifically nitrocellulose (Nitrocellulose RS-7 supplied by Daicel Kagaku Kogyo), in a lower fatty acid ester or ketone such as butyl acetate, isobutyl acetate or cyclohexanone and a film is prepared from the solution by the rotational film-forming method (see Japanese Patent Laid-Open No. 219023/83), and (3) a process in which a mixture of nitrocellulose with a plasticizer such as diethyl phthalate, dibutyl phthalate or dioctyl phthalate is dissolved in a mixed solvent comprising a dissolving agent, for example, a ketone such as acetone or methylethyl ketone or an ester such as ethyl acetate or butyl acetate, and a diluent such as xylene or toluene and the solution is formed into a film by the rotational film-forming method (see Japanese Patent Laid-Open No. 182730/84).
It was found that when a cellulose ester film is prepared from the above-mentioned starting material by the rotational film-forming method, a film having good optical properties is not always obtained.
In the rotational film-forming method, in order to prevent degradation of the thickness uniformity or reduction of the light transmission by incorporation of a foreign substance into the film and resulting flowing movement of the foreign substance, the cellulose ester solution should be filtered before the film-forming operation. However, the above-mentioned starting material is defective in that the filtering property is insufficient.